The present invention relates to self-protecting drive circuitry for electric fuel pumps. The invention arose during development efforts directed to providing a user friendly fuel pump driver circuit for use in a marine drive.
Typically, an electric fuel pump is comprised of a displacement-type pump driven by a DC permanent-magnet motor. Generally there is an integral check valve at the pump outlet to prevent reverse fuel flow through the pump. Also, a pressure regulator external to the pump outlet is normally included to regulate the pressure developed by the pump. The pressure regulator regulates the pressure by providing a pressure relieving bypass path for the excess fuel supplied by the pump, thereby preventing the pressure from rising above the desired value.
The present invention provides a fuel pump drive subsystem that is not only self-protecting but is also user friendly. A series connected semiconductor switch is automatically turned OFF in response to a given sensed abnormal or fault condition, and is automatically turned back ON after a cooling-off interval, thereby re-energizing the fuel pump, all without manual intervention from the operator. The circuitry automatically repeatedly attempts to re-energize the fuel pump while utilizing current-limiting and cooling-off intervals to protect itself. Experience has shown that these repeated re-energization attempts can actually dislodge particles of metal or rubber which may be caught in the fuel pump, and after the particle is dislodged, the pump starts running normally again.
The invention provides various stages of protection for the semiconductor driving the pump. In the preferred embodiment, a first protective stage, which is designed to sense high current through the semiconductor switch, typically senses high current occurring at initial turn-on and quickly reduces the drive signal to the semiconductor. During such reduced drive signal conditions the semiconductor does not act like a switch, but rather functions as an active current-controlling element in series with the fuel pump. The first protective stage is fast-acting and provides effective current-limiting to protect the semiconductor switch from excessive current.
A second protective stage senses abnormally high voltage across the semiconductor switch in combination with current flow through the semiconductor switch in excess of a given minimum current. Such a combination of conditions usually indicates a load fault, such as a stalled or shorted pump, that could damage the semiconductor switch if it were to remain turned ON for any significant period of time. Without counter EMF from a stalled or shorted pump there would be excessive power dissipation in the semiconductor switch. If the second protective stage detects conditions indicative of a load fault, the second protective stage quickly triggers a timer which turns OFF the semiconductor switch for a cooling-off interval. At the end of the cooling-off interval the semiconductor switch is automatically turned back ON again.
A third protective stage senses long term above-normal current flow through the semiconductor switch, symptomatic of a developing problem. The problem could be within the fuel pump itself, such as gum or varnish deposits or bearings beginning to seize, or external to the pump such as a pressure regulator stuck in the closed position, clogged fuel passages in the outlet line leading away from the pump, etc. Any of these conditions would cause the pump motor to labor against an abnormally high torque load, which would slow down the motor and increase its current demand. Should the condition continue to worsen, the dissipation in the semiconductor switch would continue to increase. Eventually the condition could deteriorate to the point that the semiconductor switch would overheat and fail. The third protective stage therefor monitors the current through the semiconductor switch, and when above-normal current has existed for a sufficient period of time, the third protective stage detects it and triggers the timer which turns OFF the semiconductor switch for a given cooling-off interval, after which it is automatically turned back ON again.
No matter which level of protection is activated, there is no need for the boat operator to go through a reset procedure, whether by the ignition key, reset button, etc. Instead, the protection circuitry automatically repeatedly attempts to re-energize the fuel pump.